Micro-Electro-Mechanical Systems (MEMS) along with a subset, the Nano-Electro-Mechanical Systems (NEMS) sensors including gyroscopes and accelerometers require very specific geometric relationships between distinct moving and stationary elements in order to function most reliably. To date, the reliability of photolithography has made photolithography the means of choice for repeatably producing the geometric relationships necessary to produce MEMS sensors.
Photolithography is the process of transferring geometric shapes on a mask to the surface of a silicon wafer. The steps involved in the photolithographic process are wafer cleaning; barrier layer formation; photoresist application; soft baking; mask alignment; exposure and development; and hard-baking.
In a first step, the wafers are chemically cleaned to remove particulate matter on the surface as well as any traces of organic, ionic, and metallic impurities. After cleaning, silicon dioxide, which serves as a barrier layer, is deposited on the surface of the wafer. After the formation of the SiO2 layer, photoresist is applied to the surface of the wafer.
Photoresist processes become geometrically more expensive as the scale of traces approach the sub-micron scale. The expense is tied to necessity of use of reduction steppers or nanoimprinters configured to work in the more demanding environment of submicron dimensions. Not only is it necessary to reproduce submicron geometries but alignment of patterns of such detail is very demanding technologically. With the greater expense, the use of reduction steppers or nanoimprinters also allows for far greater component density on the silicon die. In MEMS sensor fabrication, however, large surfaces do not require dense packing of distinct components such as transistors. MEMS trenches and beams tend to have one or two critical dimension of such components as charged plates acting as “half capacitors.” MEMS components need not be densely pack but merely require very tight tolerances and clearances for optimal performance as fabricated.
What is needed in the art is a fabrication method for MEMS components that can produce submicron trenches and beams without significantly increasing the expense of fabrication.